The field of the invention is optics, image projectors having a transparent plate and the invention is particularly concerned with the illuminating unit of a transparency overhead projector.
In conventional transparency overhead projectors, the light source is mounted underneath the projected plane or platen. As a rule the light path from the light source passes through a Fresnel lens underneath the platen and focuses the light through the art work into the projection lens.
The state of the art of the prior art projectors may be ascertained by reference to U.S. Pat. Nos. 2,529,664; 3,752,574 and 4,652,101 the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,652,101 discloses in FIG. 1 an overhead projector having a base, a transparent writing plate having on the base, a light source, objective lens and reflector supported above the writing plate, a Fresnel lens disposed below, parallel to and spaced from the writing plate, a reflector below the Fresnel lens, and a frame holding together as a unit the writing plate and Fresnel lens.
In order to achieve uniform illumination of the platen, the light source must be located relatively far from the Fresnel lens so that the light is not incident on it at too large an angle. In turn a minimum height of the projector follows therefrom, which makes such a transparency overhead projector rather bulky. Already many suggestions have been advanced to reduce that height and to create more compact projectors such as shown in U.S. Pat. Nos. 2,529,664 and 3,752,574.
Such suggestions are to the effect of placing the light source to the side fairly tightly underneath the Fresnel lens and to reflect the light by a deflecting mirror mounted below the Fresnel lens toward same and the platen.